1. Field of the Invention
The present invention relates to a sheet-fed printing press that is equipped with a double sheet detector for detecting whether two or more overlapped printing sheets are being fed, and more particularly a sheet-fed printing press that is equipped with both a photoelectric double sheet detector and an ultrasonic double sheet detector.
2.Related Art
In a sheet-fed printing press, a feeding section feeds printing sheets one by one to a printing section where printing sheets are printed one by one. For the purpose of securing printing operation by checking that printing sheets are being fed one by one, a conventional printing press is equipped with a double sheet detector that outputs a signal representative of detection of feeding of two or more overlapped printing sheets to the printing section (hereinafter simply referred to double sheet detected signal) when the detector has determined that two or more overlapped printing sheets are being fed to the printing section.
Conventional double sheet detectors include a photoelectric double sheet detector and an ultrasonic double sheet detector, the former being designed to irradiate light to printing sheets and detect such a double sheet feeding state based upon the detected difference in transmitted light therefrom, the latter being designed to emit ultrasonic sound to printing sheets and detect such a double sheet feeding state based on the detected information on whether an air layer exists. These detector are properly selected for use in printing depending on the intended use. As used throughout the description, the term “double sheet” is meant as two or more overlapped sheets when it is used relative to printing sheets.
The reason why those detectors are selected is that they each have advantage and disadvantage depending on the intended use. That is, where transparent printing sheets or printing sheets having high shielding performance (or opaque sheet) are to be processed by a photoelectric double sheet detector, the difference in transmitted light is hardly caused and therefore the photoelectric double sheet detector is unlikely to accurately detect the double sheet feeding state. On the other hand, where printing sheets such as those in the form of an envelope having front rear sheet pieces overlapped to each other are to be processed by an ultrasonic double sheet detector, the ultrasonic double sheet detector erroneously determines that a double sheet feeding state exists, due to the presence of air layer between the two sheet pieces of a printing sheet, and therefore is unlikely to accurately detect the double sheet feeding state. Therefore, both types of the double sheet detector are to be selectively employed depending on the intended use. However, the manual selection by the operator between the two types of detector based on his or her determination involves a troublesome work. In addition, the operator is sometimes hard to select a proper type of detector for use in printing when the operator relies merely on his or her visual observation. Accordingly, there is a case where the double sheet feeding detection is made by an improper type of detector, which may result in erroneous detection.
In order to address the above problem, it is known such as in Japanese Patent Laid-open No. Hei-06-263288 that, where the above two types of detector are simultaneously used and at least one of them outputs double sheet detected signal, it is determined that a double sheet feeding state exists.
However, a double sheet detector as disclosed in the above prior reference still has a problem as stated below.
That is, as described above, where printing sheets in the form of such as an envelope having a hollow portion are to be processed by the ultrasonic double sheet detector disclosed such as in the above prior reference, the ultrasonic double sheet detector still cannot overcome a problem to erroneously output double sheet detected signal even when it detected a single printing sheet and hence perform erroneous detection.
In order to address the above problem, it is an object of the present invention to provide a sheet-fed printing press that is capable of selecting a proper double sheet detector and rapidly performing printing preparatory work for more secured double sheet detection.